Update handling of exports.

tests/language/duplicate_export_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test that it is not a compile-time error to reexport the same elements
+// through different paths.
+
+library duplicate_export_test;
+
+export 'duplicate_import_liba.dart';
+export 'duplicate_export_liba.dart'; // reexports 'duplicate_import_liba.dart'.
+
+void main() {
+}